Photoreactive polymers are useful as binder resins in photoresist compositions employed in photodevelopment of electronic components such as circuit boards and other products. Circuit boards are manufactured in a number of processing steps which rely on the use of photoreactive coatings (i.e., "photoresists") that photochemically produce a difference in solubility between the photoexposed areas and the unexposed areas. In general, two classes of photoresists exist: positive acting resists and negative acting resists. A positive acting resist becomes more soluble in a developer solution when exposed to actinic radiation, and a negative acting resist becomes less soluble in a developer solution when exposed to actinic radiation.
For many applications such as in the manufacture of circuit boards, a positive acting resist is often preferred over its negative acting counterpart. One reason for this preference is the presence of "through holes" in circuit boards. The term "through holes" refers to cylindrical passages from one surface of a circuit board to another surface. Since their purpose is to provide an electrical connection between two surfaces of the circuit board, the through holes are coated with a conductive material (e.g., copper). In order to retain its conductive integrity, the conductive material lining the walls of a through hole must be protected from etchants used in forming the circuit patterns. Many in the industry use photoresists for this purpose.
In one method of protecting the conductive material lining the walls of a through hole from etchants, a preformed, positive acting photoresist film is applied over the opening(s) of a through hole. In another method, a liquid, positive acting photoresist is applied onto the circuit board such that it coats the conductive material lining the walls of a through hole.
Once the photoresist is applied, it is irradiated so as to create a solubility difference between the irradiated and non-irradiated portions. The irradiation of the photoresist, in the case of circuit board manufacture, often occurs through a glass or plastic cover sheet. Radiation passing through such a cover sheet to reach the photoresist is predominantly that having wavelengths greater than approximately 315 nanometers. The principal wavelength used for irradiation of photoresists is the 365 nanometer wavelength of a mercury vapor ultraviolet lamp. Therefore, a useful photoresist for printed circuit board manufacture is preferably sensitive to radiation having wavelengths greater than 315 nanometers, particularly to radiation in the vicinity of 365 nanometers.
U.S. Pat. No. 5,600,035 discloses photoactive monomers and polymers which use nitro substituted moieties as the basis for positive acting resist photochemistries. Specifically, in that patent, a substantial improvement in resist performance is yielded by the use of photoreactive compounds synthesized from monomers including: ##STR3## where X and Y may be the same or different member selected from the group consisting of: halogen, --OR, --O--SO.sub.2 R, --SR, --NRR', --OC.dbd.ONHR, --OC.dbd.OOR, --OSiRR'O and --OC.dbd.OR; and where R and R' may be hydrogen or any of a wide variety of organic substituents, including substituted or unsubstituted alkyl, aryl, substitutents, wherein the substitutents can be monomeric or polymeric. If further reaction to form adducts or polymers is desired, the R or R' groups may include a reactive group such as a hydroxyl group. After exposure to radiation such as ultraviolet (UV) light, the bond is broken between the carbon and the X in the CH.sub.2 X group, thus providing photoactivity to the compounds.
According to U.S. Pat. No. 5,600,035, the 2,6-dinitro structure (1) is preferred due to its high degree of photosensitivity. Thus, preferred embodiments of intermediates and polymers may be derived from the monomer: ##STR4## where Q is halogen and/or --OH. Also according to U.S. Pat. No. 5,600,035, the 2,6-dinitro-1,4-bis(dichloromethyl)-benzene species of structure (3) has been found to be particularly useful, and the corresponding diol species can be derived from the dichloro monomer. Both the chloride and hydroxyl groups are reactive with a wide variety of substances whereby intermediates and polymers can be synthesized from the dichloro or diol monomers of structure (3) or from the corresponding 2,5-dinitro monomers. The oligomers or polymers thus formed are highly photoreactive and find use as positive acting photoresists and the like. These polymers and oligomers also form part of the invention recited in U.S. Pat. No. 5,600,035. Those oligomers or polymers include the photoreactive groups as defined above and at least one ether, ester, urethane, carbonate, thio, or amino group or combinations thereof. Each of these substitutents may include a reactive group (e.g., OH) to enable further reaction or copolymerization, if desired.
U.S. Pat. No. 5,600,035 discloses that polymers can be prepared from monomers and intermediates having the defined bis(chloromethyl)dinitrobenzene or dinitrobenzene dimethanol structures which are hydrolytically and thermally stable to the processing conditions required for photoimaging, such as in the manufacture of circuit boards. That patent also discloses that polymers such as polyurethanes, polysulfides and polyethers can be produced and are known to be stable in electrocoating baths; and that polyesters, polyamines and polyquaternized amine polymers have also been prepared with the desirable dinitro groups. Optionally, the photoreactive polymers may include salt forming groups or may be blended with another polymer that has salt forming groups to permit aqueous dispersion and electrodeposition of the photoresist composition onto conductive substrates.
Novel, high yield syntheses of the aforementioned dinitrobenzene monomers having functional substitution at the 1,4 positions are also part of the invention recited in U.S. Pat. No. 5,600,035, as well as methods of making intermediates and polymers from those monomers. Notwithstanding the improvement in the art provided by U.S. Pat. No. 5,600,035, there is always a desire to improve a photoresist's photospeed and/or contrast ratio. As used herein, the term "photospeed," as it pertains to a photoresist, refers to the integrated dosage of actinic radiation (typically expressed in milliJoules/cm.sup.2) required to effect a usable image transfer into the photoresist film. Also, the term "contrast ratio," as it pertains to a photoresist, refers to the difference in solubility between the exposed and unexposed portions of the film in an appropriate developer solution. The contrast ratio is a unitless number most easily calculated from the amount of time it takes a developer solution to completely dissolve unexposed film divided by the amount of time required to dissolve the exposed portion of the film. For positive acting resists, the contrast ratio is always greater than one with higher numbers being preferred.